During conventional semiconductor circuit manufacturing, hot ultrapure water is often employed to rinse the circuits being manufactured. It is impractical to purify the water after it has been heated. Accordingly, the water is first purified, and then heated just prior to use. Hot ultrapure water and other ultrapure fluids are employed for many other conventional, commercially useful purposes.
One type of conventional water heating system (described, for example, in U.S. Pat. No. 1,807,951, issued to Ahern on June 2, 1931) includes an elongated chamber which has an annular cross-section. The chamber surrounds an elongated heating element which extends along the chamber's axis. The heating element heats water within the chamber while the water flows axially along the chamber from an inlet to an outlet.
However, this type of conventional heating system does not confine the water being heated within a chemically inert container. Thus, this type of system may contaminate the water due to contact with the chamber itself, or in the event of failure of the chamber, with the heating element or other system components. For this reason, such conventional systems are unsuitable for heating ultrapure water (or other ultrapure fluid) to the high temperatures typically required for use in semiconductor circuit manufacturing, while maintaining the purity of the fluid being heated.
Systems for heating ultrapure water have been proposed. For example, U.S. Pat. No. 4,461,347, issued to Layton, et al. on July 24, 1984, describes such a system in which ultrapure water flows axially within an annular volume between an outer tube and an inner coaxial tube. The coaxial tubes are composed of (or coated with) an inert, non-reactive material such as polytetrafluoroethylene (known as "TEFLON" material). Heat is conducted from hot liquid within the inner tube (or an electrical heating element within the inner tube) to ultrapure water flowing axially within the annular volume. Use of this type of heat exchange mechanism has a number of disadvantages, including the following.
The heat exchange mechanism relies on heat conduction between the inner tube (which may have an inert coating) and flowing ultrapure water. The ultrapure water is in direct contact with the inner tube (or its coating), so that there is a risk that the tube or its coating will fail, thereby exposing the ultrapure water to contaminants.
Furthermore, inert heat source coatings (such TEFLON) in direct contact with ultrapure water (as in the system of U.S. Pat. No. 4,461,347) are suitable only for use within a limited temperature range. A TEFLON material coating, for example, will melt or burn if raised above a critical temperature (typically in the range from about 250 to about 300 degrees Celsius). Thus, conventional heat exchange systems of the described type are unsuited for use with heat sources which would raise the heat source coating temperature above the critical temperature.
Finally, the conductive heat transfer process employed in conventional heat exchange systems of the described type is much less efficient than the radiative heat transfer technique employed in the present invention. Conventional heat exchange systems are particularly inefficient in relation to preferred embodiments of the invention employing a quartz coil with infrared heating lamps which efficiently radiate wavelengths within the high transmissivity "window" of the quartz coil.
Some conventional water heating systems of the heat exchange type heat a volume of liquid, and the heated liquid in turn conducts heat to ultrapure water flowing within adjacent chemically inert tubing (for example, PVDF plastic tubing). In addition to the above-described disadvantages and limitations of heat exchanging systems in general, these systems have employed chemically inert tubing with an undesirably large heat exchange surface area. Use of tubing with such a large surface area undesirably promotes contamination of the water flowing within the tubing due to bacterial growth and the like.
Until the present invention, it had not been known how to design and operate a fluid heating system employing radiating lamp heating elements in a manner eliminating the above-described disadvantages and limitations of conventional systems.